1. Field of the Invention
The present invention relates to a DC current sensor utilized in a wide range of field such as preventive maintenances of installations by leakage detections of, DC generating apparatus such as a solar cell, fuel cell generating system and the like, DC control circuits and the like of a power plant, a substation and a large switchboard for industrial plant, and further, insulation deterioration in various DC equipments, more particularly, it relates to a sensitive DC current sensor having a relatively simple construction and a good detecting capability for even a micro-current variation, realizing a stable detection.
2. Description of the Prior Art
In recent year, though equipments utilizing a DC current are used in a wide range of field, demands on a sensor for detecting load of a DC motor for necessary control and a DC current sensor used in a DC current leakage breaker and the like are enhanced in maintenance for operating the various equipments smoothly.
As such DC current sensors, those consisting of a magnetic amplifier type, magnetic multi-vibrator type (Japanese Patent Application Laid Open Nos. Sho 47-1644, Sho 53-31176, Sho 49-46859), Hall device type and the like are well known.
Both the magnetic amplifier type and magnetic multivibrator type use a core of soft magnetic material constituted by winding a detecting coil in a toroidal shape, wherein by extending a lead wire being detected through the core for DC magnetic deflection of the core of soft magnetic material within a saturated magnetic flux density (Bs) by a DC current flowing through the lead wire being detected, an alternating magnetic flux produced by applying an AC current to the coil wound around the core beforehand produces an unbalance when saturated in the positive and negative directions, and the variation is detected by the detecting coil. Since the magnetic flux variation is given in the core beforehand in the former type, a configuration of applying the AC current of a predetermined value by winding the excitation coil around the core is adopted, in the latter type, it is so constituted that the self-excitation takes place by the action of semiconductors and the like in a circuit connected to the detecting coil, and a duty ratio of an oscillation waveform is changed responsive to the current being detected for oscillation.
Furthermore, the Hall device type is so constituted that, the lead wire being detected is wound in a toroidal shape directly around the core of soft magnetic material which is formed partly with a gap disposing the Hall device, and the magnetic flux variation in the core responsive to the DC current variation flowing through the lead wire being detected is detected directly by the Hall device.
However, it is the present situation that, the DC current sensors of the above-mentioned types are not always constituted to respond to the micro-current variation of a DC leakage breaker and the like by the following reasons, the they are not being used practically as the sensitive DC current sensor.
In the magnetic amplifier type and magnetic multivibrator type, as previously described, the core of soft magnetic material must be subjected to DC magnetic deflection by the DC current flowing through the lead wire being detected so as to saturate nearly to the saturated magnetic flux density (Bs), resulting in a low detecting sensitivity. And hence, in the case of using the well known soft magnetic material such as permalloy and the like as the core, for example, when the current flowing through the lead wire being detected is about several tens of mA, the lead wire being detected must be wound around the core of soft magnetic material by several tens to several hundreds of turns or more, thus it was difficult to use as the DC current sensor for the leakage breaker and the like where the lead wire being detected is required by one turn.
Also in the Hall device type, since the detecting capability is determined inevitably by the characteristic of the Hall device, when the Hall device which is known at present is used, for example, when the current flowing through the lead wire being detected is about several tens of mA, the lead wire being detected must be wound around the core of soft magnetic material by several hundreds to several thousands of turns or more, thus as same as the aforementioned magnetic amplifier type and magnetic multivibrator type, it was difficult to use as the DC current sensor of the leakage breaker or the like where the lead wire being detected is required by one turn.